This invention relates to producing dilactic acids or dimers more efficiently by specifically producing aminium lactate salts from lactic acid fermentation broths, e.g. cheese whey. The salts are then thermally dissociated in an inert gas stream at low temperatures in the presence of a cyclization catalyst to form dilactic acids (dimers or diesters). The resulting dilactic acids or diesters may then be used to produce biodegradable polymers such as polylactic acid, and cosmetics and pharmaceuticals.
Lactic acid is currently processed into polylactic acid which can be used in many biodegradable polymer applications. Polylactic acid is a multi-functional thermoplastic which can be processed into staple fibers (e.g. carpet fibers), spinning fibers in woven applications to replace (or in blends with) cotton, wool, and polyesters, extruded films for wrappings, injection and thermo-molded products such as polyethylene, propylene and styrene foam products, and thermo-formed plastics such as eating utensils, coatings, etc. Polylactic acid is completely recyclable and is the only major polymer which slowly yet totally biodegrades during composting.
The use of polylactic acid as a mass polymer, until now, has been limited due to the high costs associated with its production, primarily energy costs, making it uncompetitive with similar non-biodegradable petroleum-based polymers and polyesters. There are two major routes to producing polylactic acid directly from the lactic acid monomer. The first route involves removal of water of condensation by using a solvent under high vacuum and temperature. This approach is currently used, for example, by Mitsui Toatsu Chemicals to produce a low to intermediate molecular weight polymer. In a second alternative route, which is considered to be the classical approach to producing polylactic acid, water is removed under milder conditions directly from lactic acid, without solvent, to produce a cyclic (ring closing) intermediate dimer referred to as xe2x80x9cdilactic acid.xe2x80x9d This dimer is then purified under vacuum distillation and then xe2x80x9cring-openingxe2x80x9d polymerization is accomplished using heat, without solvent, to produce polylactic acid. This xe2x80x9cring-openingxe2x80x9d method of producing polylactic acid is currently used worldwide and is the subject of many patents and other literature. This process, however, suffers from long reaction times and high temperatures and the formation of a number of side reactions and by-products. It usually results in a low (50%-55%) chemical yield for the polylactic acid polymer.
Recently (1992) a third route of producing polylactic acid has been patented and is now being commercially practiced by Cargill, see U.S. Pat. No. 5,142,023 of Aug. 25, 1992. This process relies on the initial production of an impure polylactic acid polymer as a feedstock in the production of polylactic acid. This impure polymer must then be depolymerized using additional energy steps in order to achieve a more pure polylactic acid polymer. These steps are also energy expensive and therefore result in a high production cost associated with producing polylactic acid.
Yet another process of producing dilactic acids or dimers and subsequently producing polylactic acid avoids such energy intensive steps as described in the Cargill process of U.S. Pat. No. 5,142,023. This particular method uses an aminium lactate salt (crystal) instead of an impure polylactic acid as a starting material in the production of dilactic acids or dimers. It is described in Kamm et al, Formation of Aminium Lactates in Lactic Acid Fermentation, Acta BioTechnol. 17, (1997) 1, 3-18. It describes the use of organic amines (technically called heterocyclic amines, e.g. Piperazine) within the lactic acid fermentation broth to produce aminium lactate (salts). Though aminium lactate salts are referred to specifically, other salts such as ammonium lactate salts may also be produced and used in such a process. Aminium lactate salts have lower melt points of from 80xc2x0 C.-150xc2x0 C. and can dissociate in the presence of catalysts (acetonitrile, dioxan, ethylene glycol monoethylether, dimethyl sulphoxid-d6) and low heat to form dilactic acids or dimers. This process completely avoids the need to first produce impure polylactic acid polymers as the feedstock in order to produce such dilactic acids. In this process, however, ultrafiltration and electrocoagulation are used to concentrate and extract the lactic acids and lactate salts. The fallacy of this process, for large scale processing, lies in the use of the organic amines within the fermentation broth and the use of ultrafiltration membranes that require high-energy pressures to remove and separate out the cell mass from the lactic acid/aminium salt. Once the lactic acid is separated from the cell mass, electrocoagulation is then used to bring about the separation or breakdown of the lactic acid from the amine salt in order to concentrate it to a minimum of a 45-85% pure lactic acid. The purer lactic acid is then re-contacted with the organic amine once again, e.g. Piperazine, to form the Piperazine salt once again. In this method the ultrafiltration membranes requires high-energy pressure during operation and often becomes fouled and plugged. It is therefore not suitable for commercial scale processing. In addition, the electrocoagulation step is also not scaleable for commercial use. The Kamm process, as described, requires unnecessary steps of forming the salt from the lactic acid in order to achieve a higher concentration of the lactic acid (45-85%), which then must be re-crystallized to form the salt. This then must be destructured to form the dilactic acid or dimmer. This process results in the production of impure dilactic acids and aminium lactate salts (as an interim step), and the impurities in the lactic acid produced during fermentation within this process has limited the polymer length achievable, mainly due to such lactic acid refining techniques practiced and described in the Kamm process.
It has now been discovered that the aminium lactate salts of the Kamm process can, under certain conditions, become a low cost and low energy step in the production of polylactic acid. The present invention, unlike the Kamm process, does not rely on the concentration of lactic acid to 45-85% purity and thus represents a major energy saving step within the present invention.
Accordingly, as can be seen from the above description, a new and more efficient method is needed for the production of dilactic acids (dimers) that produces interim aminium lactates (salts) more efficiently than that disclosed in the so-called xe2x80x9cKamm processxe2x80x9d. Such aminium lactates (crystals) could therefore become a new, low-cost feedstock for the production of dilactic acids or dimers. These salts could then be processed more efficiently and with less energy cost to produce such dilactics (acids) or dimer esters and subsequent polylactic acid polymers more economically. It is a primary object of this invention to fulfill this need.
Besides fulfilling this need for a new route to producing dilactic acids (dimers) and subsequent polylactic acid polymers as described, several additional objects and advantages of the present invention are:
(a) to provide a less capital and energy expensive new route to producing dilactic acids (dimers) and subsequent polylactic acid polymers that relies on producing such dilactic acids (dimers) directly from aminium lactate salt (crystal) rather than impure polylactic acid through ring-closing catalysis. Though aminium lactate salts are specifically discussed within the present invention, the present invention is not limited to the use of such specific salts but may include other earth and alkaline salts which may also be produced and used accordingly. Therefore, where mentioned, aminium lactate salts refers also to such other salts as well;
(b) to provide a less expensive and more efficient method to produce aminium lactate salts using methods that are readily commercially scaleable;
(c) to provide a less expensive feedstock in the form of aminium lactate salts that when used in the manufacture of dilactic acids (dimers), and subsequent polylactic acid polymers, will reduce the cost of producing such dilactic acids (and subsequent polylactic acid polymers);
(d) to provide a product in the form of aminium lactate salt that is more readily purifiable over impure polylactic acid polymers and that when used as a feedstock in the manufacture of dilactic acids (dimers), will require less energy, less purification and less process time to dissociate in order to produce such dilactic acids (dimers), and subsequent polylactic acid polymers, therefrom;
(e) to provide a two-step method or process using two membranes in sequence to produce such aminium lactates wherein such method is readily commercially scaleable in order to produce aminium lactate salts more efficiently for use in producing dilactic acids (dimers);
(f) to provide a single step membrane process wherein such process can be readily up-scaled and commercialized to produce aminium lactate salts more efficiently;
(g) to provide a method that uses various lactic acid fermentation broths, or other more concentrated lactic acid solutions, with a highly efficient method of fermenting lactic acid such as is shown in U.S. Pat. No. 5,563,069 entitled: xe2x80x9cExtractive Fermentation Using Convoluted Fibrous Bed Bioreactorxe2x80x9d combined with a novel method of extracting such fermented lactic acid directly across a membrane into a strong pH base organic amine, e.g Piperazine in solution, as the contacting solution, to form organic amine salts (e.g. aminium lactate salts or other earth or alkaline salts such as ammonium lactate salts). This method further reduces the processing steps, energy costs and subsequent capital costs in the production of polylactic acids directly through employment of the method of this present invention; and finally,
(h) to provide a cost competitive method of using organic, renewable products and environmental wastes, e.g. cheese whey, sawdust, forest debris, brush, crop residues, municipal solid wastes, food processing wastes from which to produce dilactic acids (dimers) and subsequent polylactic acid.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing written description of a preferred embodiment.
A new method of producing dilactic acids (dimers) by first producing aminium lactate salts by means of membrane technology to extract lactic acid from one side of a membrane to another to form aminium lactate salts, and thereafter using such salts as the starting material or feedstock in the manufacture of such dilactic acids or dimers. In this way, by contacting lactic acid across a membrane with an organic amine solution (e.g. Piperazine) to yield aminium lactate salts, which can then be dissociated in the presence of an inert gas at low temperature and in the presence of a cyclization catalyst (ring closing) to form dilactic acids or dimers, lengthy, expensive process steps are avoided. The dimers which may subsequently be polymerized to polylactic acid using conventional technologies.